1. Field of the Invention
The invention relates to an exhaust gas recirculation apparatus for an internal combustion engine and a method of controlling the exhaust gas recirculation apparatus.
2. Description of the Related Art
An exhaust gas recirculation apparatus for an internal combustion engine is available. The exhaust gas recirculation apparatus includes a turbocharger and a low-pressure EGR passage. The turbocharger includes a turbine provided in an exhaust passage, and a compressor provided in an intake passage. The low-pressure EGR passage connects an area of the exhaust passage downstream of the turbine to an area of the intake passage upstream of the compressor. EGR gas, which is part of exhaust gas discharged from the internal combustion engine, is recirculated to the intake passage through the low-pressure EGR passage. Another exhaust gas recirculation apparatus that includes a high-pressure EGR passage is available. The high-pressure EGR passage connects the area of the exhaust passage upstream of the turbine to the area of the intake passage downstream of the compressor.
Japanese Patent Application Publication No. 2004-150319 (JP-A-2004-150319) describes that the low-pressure EGR passage or the high-pressure EGR passage is selected based on an engine speed and an engine load, and the EGR gas is supplied through the selected EGR passage.
However, when a vehicle starts accelerating after the vehicle has decelerated or fuel supply to the engine has been cut off, an air-fuel ratio may temporarily increase, and NOx may be discharged. When the fuel supply is cut off, air flows into the engine cylinders. Therefore, the air-fuel ratio of the gas in the intake pipe, cylinder, or exhaust pipe gradually increases. Accordingly, when the vehicle starts accelerating after the vehicle has decelerated or the fuel supply to the engine has been cut off, the supplied EGR gas is almost the same as the atmospheric air. Therefore, the EGR gas may not effectively suppress the generation of NOx. Thus, when the vehicle starts accelerating, NOx may be discharged.
Also, when the vehicle is decelerating or when the fuel supply to the engine is cut off, the rotational speed of the turbocharger decreases. As a result, a boost pressure may decrease. When the accelerator pedal is depressed after the boost pressure has decreased, the acceleration of the vehicle may be slow.